Some months ago we were receiving a bad sewer smell in our basement. The smell was coming directly from the floor drains. Upon investigation we determined that when our relief system was installed and connected to the main sewer line, that P-traps were not installed at the drains. We had a plumber/contractor come out and evaluate the problem. New PC piping and P-traps were installed at and between each drain in the basement. The smell problem had been corrected, or at least it went away until just recently. Every time we run the washing machine, the sewer gas smell returns, although it doesw not seem to be coming from the drain in the floor. Someone mentioned that there may a problem with trapped gas. Can anyone tell me what the problem might be and how it may be corrected?